This invention relates to an apparatus for testing the wall thickness of a tube of electrically non-conducting material produced by an extruder.
The apparatus according to the invention comprises an electrically conductive measurement body held within the moving tube and a measuring sensor situated outside the tube within effective range of the measurement body, and arrangements of this general type are described in, for example, Swiss Pat. No. 563,567. In most cases, the measurement body is located within the tube by attachment through some kind of rod to the mandrel of the extrusion nozzle, and thereby held in a predetermined axial position. This construction has the disadvantage that the measurement body must be fixed to the nozzle of the extruder before the latter is brought into operation, which complicates the operation because the measurement body is then immediately in the path of movement of the leading end of the tube issuing from the nozzle.
It is also known to introduce the measurement body freely into the tube to be measured and, by means of a magnet, to hold the measurement body in the desired axial position and against the wall of the tube. However, in this case the measurement body consists of a single rigid component which introduces the risk that, under the magnetic attraction which influences the measurement body from the side where the measuring sensor is situated, the measurement body will, on encountering irregularities or curvatures in the tube, bear tightly against the tube wall in the region of the holding magnet but have a certain amount of clearance from the tube wall in the region of the measuring sensor.
It is an object of the present invention to allow the measurement body to be held within the tube so that it will always be reliably seated against the inside of the tube, and to allow the introduction of the measurement body into the tube simply and without disturbing the production process.